Acute pulmonary embolisms (PEs) occurs when venous thrombi are dislodged and travel through the vascular system and partially or completely occlude the pulmonary artery. There are no clinical signs or symptoms that are specific for PEs because many of the presenting features are common among patients without PEs, rendering this diagnosis challenging. PEs represent a relatively common cardiovascular emergency with about 600,000 cases occurring annually, causing approximately 200,000 deaths in the US. According to one retrospective study covering a 25-year span, the annual incidence of pulmonary embolism is 69 per 100 thousand. In another study, the PE incidence rate in the Worcester Statistical Metropolitan Area was reported to be 27%. Pulmonary embolism was thought to be the cause of death in 239 of 2388 autopsies performed. The unsuspected or undiagnosed ante mortem PE rate at autopsy may be as high as 78% (1902/2448). With early diagnosis and treatment, the mortality rate can be reduced to less than 11%.
Pulmonary embolisms do not have consistent sizes or morphologies, and the affected pulmonary arteries may be obstructed completely or partially. When embolic obstruction is complete, the entire arterial cross-section is occluded with embolic material causing the normally enhanced vascular lumen to show low attenuation. When partially occlusive embolic disease is present, enhanced blood partially surrounds intravascular emboli creating the appearance of low-attenuation intravascular material either seen floating within the vessel lumen or in contact with the lumen wall.
Central PEs may be located in the main, left or right pulmonary arteries. Detecting central PEs is critical because the major pulmonary arterial obstruction can significantly compromise pulmonary perfusion and impair right ventricular function and result in sudden death. In one study, 579 patients were evaluated and, in 516 hemodynamically stable patients, central localization of emboli was an independent predictor of all-cause death or clinical deterioration while distal emboli were inversely associated with these outcome events. In a study on detection of central PE on unenhanced CT, of the 88 studies positive for pulmonary emboli, 64% had central involvement. In another study, clinically “silent” PE—an unexpected postmortem finding—was found in 32% of patients and may affect the central pulmonary arteries. Saddle PE, which may be in the main artery, was diagnosed in 17 (5.2%) cases of 289 patients with acute PE confirmed by spiral CT. In one study, 12.1% of 214 examinations had central clots. In a landmark study that initially described the CT technique now widely used for suspected pulmonary embolism assessment, 37% partially and 46% completely occlusive pulmonary arterial filling defects were found in 42 patients with central PE.
The primary imaging technique utilized for PE diagnosis is computed tomography pulmonary angiogram (CTPA), where PEs appear as a low-attenuation filling defects within the normally highly attenuating, enhanced pulmonary arteries. In particular, a “dark” or hypo-attenuating region within a pulmonary artery represents a region of the vessel where intravenous contrast was excluded, indicating the presence of an PE candidate. Analysis by a radiologist typically involves the careful tracing each branch of the pulmonary vasculature in order to look for and identify suspected PEs. Although it is easier to detect central PEs compared to sub-segmental PEs, for instance, central PEs are often overlooked by human readers. This is because of the large number of arteries that have to be tracked as well as complexity and number of the images, with typical CTPA datasets having up to 500 individual images. Hence, PE diagnosis often requires extensive reading time whose accuracy depends on a clinician's experience, attention span, eye fatigue, as well as sensitivity to the visual characteristics of different PEs.
Computer aided diagnosis (CAD) systems for PE detection have been utilized as a “second pair of eyes” in the clinical setting, in the attempt to improve the radiologists' performance. However, computer-aided detection is a challenging task because the images are complex and include artifacts, and vary in imaging conditions. CAD systems for PE detection are typically focused on segmental and sub-segmental emboli, with only very few being capable of detecting central emboli. This is because such systems generally apply a lung segmentation that excludes the main, left and right pulmonary arteries and, as a result, central emboli are likely missed. For example, the CAD product developed for Philips is unlikely to be capable of detecting large or central emboli. The system developed by Siemens detects 88.5% of emboli affecting central pulmonary arteries.
In one study, a computationally expensive methodology was used to segment the pulmonary trunk, left and right pulmonary artery to detect central PE from 33 cases, achieving a 14.4 FP/ds rate with 95.1% sensitivity. In another study, an active contour model was used to extract the pulmonary arterial tree, employing a k-means algorithm and the Gaussian mixture models for main vessel searching and using a skeleton model for vessel tracking. The detection system employed a relatively sophisticated algorithms, and achieved a 3.8% FP/ds overall performance rate, but with a small sample size of only 16 cases. In yet another study, an expectation-maximization analysis was used. The vessel tree was reconstructed and a rule-based method was used to detect PE. An initial seed point was used to initiate vessel tracking. Only 14 cases were tested and the detection sensitivity was reported as 67% for the pulmonary main, lobar, and segmental arteries. As appreciated, these studies were limited to sample sizes, hindering a solid performance evaluation.
Thus, there remains a continuing need for new technologies capable of detecting pulmonary arterial emboli with high sensitivity and reliability.